     9 ; Metronome button
     (when (pos? (:velocity message))
       (enter-metronome-showing controller))
+
+    ;; 28 ; Master button
+
+    29 ; Stop button
+    (when (pos? (:velocity message))
+      (enter-stop-mode controller))
 
     49 ; Shift button
     (swap! (:shift-mode controller) (fn [_] (pos? (:velocity message))))

  {:pre [(<= 0 line 3)]}
  (midi/midi-sysex (:port-out controller) [240 71 127 21 (+ line 28) 0 0 247]))

(defn- show-labels
  "Illuminates all buttons with text labels, for development assistance."
  ([controller]
   (show-labels controller :bright :amber))

    (reset! (:next-text-buttons controller) {})
    (Arrays/fill (:next-top-pads controller) 0)

    (update-effect-list controller)
    (update-metronome-section controller)

    ;; Reflect the shift button state
